I just ordered the above lens, to test it thoroughly. Resolution figures at the borders are not looking too good here on a 8MPix Canon, and the resume sounds like this lens is a real loser. Well, perhaps its better to have a well-centred medium-res lens than a potentially super-duper lens with massive decentering defects...
I'll compare it to a second copy of Sigma's 10-20mm as the first copy of that lens had a brutal decentering defect, as you can see here.
Plus a Sigma 12-24mm and a Tokina 12-24mm.

Now on to some test-results!
The material I shot is not as broad as with the Sigmas, as the Tamron seems to be lacking from the very first shots. But you have to be careful not to throw the baby out with the bathtub. There are some good sides of this lens picture-wise in addition to it being the cheapest and lightest of the tested lenses and the second shortest.
So here we go:
My favorite shot is trees against the sky as the harsh contrast easily shows color-fringing and the fine structures are a good measure for sharpness. They are also sufficiently far away for a wide-angle lens so as not to bother about focus. Following is a shot taken at 11mm f8 1/125sec with two red rectangles marking the places where the 100% crops were taken:
Crops reproduced at 100% (259x387)
|---------------------centre---------------------||---------------------corner---------------------|
As you can see, the centre crop is quite sharp and "fringe-free" but the quality deteriorates markedly in the outer 10% of the pic. You see strong color-fringing and the evaluation of sharpness is a little influenced by this. But if you look at the twigs from the leaves, you can see some of them quite clearly. So perhaps sharpness is not too bad were it not overlayed by fringing...
But remember that this a shot at f8. In the next section we'll scrutinize sharpness (and fringing) at wide open aperture, that should turn the heat on a little...

Let's have a look now on a "lab"-shot featuring the famous "Siemens-stars" and taken at 11mm f4.5 1/3000sec:
Followed again by a 100% centre-crop and crop from the upper right corner (which is not the same corner as in the previous pictures because we're are now shooting in landscape format!)
Once again you see strong color fringing at the corner-crop but now we can start to look also at sharpness. For this you have to concentrate on the center of the Siemens-star and evaluate the diameter of the inner-most zone, where the individual lines blur completely to form a "grey disk".
When I put my ruler to the screen to measure the diameter of the grey disk I get 9-12mm at the corner-crop and around 6mm at center-crop. That means: corner-resolution of this lens at 11mm wide open is only 50-67% of centre-res DISCLAIMER: On your screen you may measure other absolute values, but the ratio should be similar!
If you look closely at the grey disk of the corner-crop you can clearly see that it is not a perfect circle but more of a skewed figure 8. That means that a line running from the centre of the whole pic through the Siemens-star in the corner (sagittal) is resolved (30%) sharper than a line rectangular to it (meridional) This is why you see two different lines in most MTF-diagrams, the sagittal normaly solid and the meridional line dotted If someone wants to impress you with the quality of only one of those lines, forget it! Always look at the values in a MTF-diagram that are the minimum of the two lines: because you never know which way the twigs are oriented

Now onto the next part. A full close-up shot of the Siemens-star, to determin the IQ under these circumstances. I often wonder why nobody is showing lens-quality at magnification of near 1:10. So here is a world first (at least at Camera Labs ):
If you have your ruler still handy and click through to the original size, you can measure the grey disk in the centre star at approx. 6mm again. So no big deal here: centre-sharpness for close objects is as good as sharpness at infinity. If you measure the little stars in the corners (well not really in the corners) you get around 7-8mm grey disk with not much deviation from corner to corner (which is proof that I had the camera adjusted well and the lens is not obviously decentered.

Well now up to the last test: The "white-dwarf". What am I trying to accomplish here? I just wanted to show you how sensitive this lens is to contra light under controlled (well, almost controlled) conditions. So here we go...
What did I do? Pull down the shades, picked up my trusty olde mini Maglite LED and shone it at different angles directly into the lens. Before, I made sure that the exposure of the otherwise dark room was not influence by the "white dwarf". What I'll show you is a typical and a worst case and make some comments on what you can expect. The full gory details are in flickr, wibble over here. I made shots at 11mm and 18mm wide open. If you close the aperture, some of the ghost images become more pronounced and sharper but not so large (some of them are images of the aperture itself!).
Well, I was positively surprised, ghost and flare were no big deal with this lens, even when shooting straight in the "sun" The first shot is @11f4.5, the second @18f5.6.

Summary--------------------------------------------------------------

Would I buy this lens? Well, not really! The fringing in the corners it pretty prominent, there are better lenses out there (the Sigma 10-20mm). If you think about investing 360€ in this lens, think again: The Sigma is only 460€...

Can you make great photos with this lens? Sure! Great photos don't depend on color-fringing! The one shot that I love most with this lens is here, although it's far from beeing a "great" photo!

Ah well, I forgot about geometric distortions and light fall-off: that's because I definitely would not decide for or against a lens based on these two factors. Light fall-off can be viewed at my photos and for geometrically inclined there are two very special shots: @11mm f4.5 and @18mm f16. They also show corner-(un)sharpness quite beautifully. Have fun!

So that's it boys and gals! Phew, I'm pretty much exhausted myself. Now start the Flak...

Excellent review and analysis Thomas! Good work, and much appreciated...

It's interesting, I am currently testing the Sony DT 11-18mm for a forthcoming review and as you know it bears some uncanny resemblances to the Tamron 11-18mm. I asked Tamron and Sony and both tried to assure me that neither company produces lenses for the other - but surely the evidence begs to differ!

Anyway, I guess it will be speculation or a slipped remark from a supplier one night!

Either way, the Sony 11-18mm can also be soft in the corners... it'll be interesting to compare your results against mine for the Sony when I publish the review in the near future.